There exist aqueous solutions of H.sub.2 SiF.sub.6, such as waste streams from processing of ores or minerals that contain silicon and fluorine values, which would be useful in chemical processes, such as in preparing fumed SiO.sub.2 and HF, if the H.sub.2 SiF.sub.6 could be purified to a constant quality by distillation. Attempts to distill water or other low-boiling material from relatively weak solutions of H.sub.2 SiF.sub.6, in order to concentrate or purify the H.sub.2 SiF.sub.6, cause decomposition and create unwanted forms of SiO.sub.2. Attempts to distill H.sub.2 SiF.sub.6 away from higher boiling impurities, such as H.sub.3 PO.sub.4 or H.sub.2 SO.sub.4, also cause decomposition and create unwanted forms of SiO.sub.2. The SiO.sub.2 so formed has no significant commercial value, thus the commercial significance of H.sub.2 SiF.sub.6 is substantially negated by the formation of relatively useless forms of grainy-type SiO.sub.2. If the aqueous H.sub.2 SiF.sub.6 can be converted to a relatively constant quality and concentration, it can be used in processes which form commercially significant products, such as being combusted to make fumed SiO.sub.2 and HF. Fumed SiO.sub.2 is a form of SiO.sub.2 which has good commercial value.
There exists then, a need for converting aqueous solutions of H.sub.2 SiF.sub.6, especially those solutions which are not of constant quality or concentration, to solutions which can be distilled to relatively constant quality and concentration without encountering the decomposition of H.sub.2 SiF.sub.6 and without creating unwanted forms of SiO.sub.2.
As further background to certain embodiments of the present invention, there are a number of references which teach the burning of silicon fluoride compounds, such as SiF.sub.4 and H.sub.2 SiF.sub.6, by injecting the silicon fluoride compounds into a combusted mixture of fuel and oxygen in order to prepare fumed SiO.sub.2. The oxygen is generally supplied as air and, conveniently, the fuel may be hydrogen. The fuel also may be an easily combusted hydrocarbon such as an aliphatic hydrocarbon having from 1 to about 12 carbon atoms or mixtures of such hydrocarbons. Representative patents for such formation of fumed SiO.sub.2 are, for example, U.S. Pat. Nos. 2,819,151; 3,203,759; 3,110,562 and others. It is highly desirable that the feed stream of H.sub.2 SiF.sub.6 be of substantially constant quality and concentration in order that the process not be erratic, especially since such processes are normally expected to be performed continuously.
Throughout this disclosure, the "decomposition" which is avoided by the present invention is illustrated by: EQU 3H.sub.2 SiF.sub.6 (aq.) .sup.vaporization 3SiF.sub.4 (.uparw.) + 6HF(.dwnarw.),
then in the distillation condenser: EQU 3SiF.sub.4 + 2H.sub.2 O(1) .fwdarw. SiO.sub.2 (grainy)+2H.sub.2 SiF.sub.6.
it is an object of the present invention to provide an aqueous solution of H.sub.2 SiF.sub.6 which is of substantially constant quality and concentration for use as feed material to chemical processes.
It is another object to provide an aqueous solution of H.sub.2 SiF.sub.6 which can be distilled without encountering decomposition and without creating unwanted forms of SiO.sub.2.
It is still another object to provide an aqueous solution containing H.sub.2 SiF.sub.6 of substantially constant quality and concentration for use in a high temperature combustion process in which fumed SiO.sub.2 and HF are produced.
Another object is the removal of substantial amounts of water from weak solutions of H.sub.2 SiF.sub.6 by distillation at elevated temperature to make a more concentrated solution without encountering decomposition and creation of grainy-type SiO.sub.2.
These and other objects are obtained by the invention described and claimed herein.